In many applications, it is necessary for the motor to turn in either direction as required, and various reversible motor drive control circuits have been proposed heretofore. For instance, in a passive seat belt system for a motor vehicle, one end of the seat belt is fixedly secured to the vehicle body but the other is required to move between a first position for restraining the vehicle occupant and a second position for relieving the occupant from the restraint of the seat belt, according to the intention of the occupant to get on or off the vehicle. The intention of the occupant may be detected by a switch for detecting the closing and opening of the vehicle door, a switch for detecting pressure upon the seat, and other means. Typically, the movable end of the seat belt is actuated with an electric motor, and the activation and the rotational direction of the motor are both required to be controlled as required.
U.S. Pat. No. 4,410,016 discloses a control circuit which comprises a timer circuit for keeping a motor rotating for a predetermined period time. This time period is longer than the time period which is normally required for the movable end of the seat belt to travel from one end of the stroke to the other, and the rotational direction of the motion is determined depending on whether the door is open or closed. Therefore, according to this proposal, the wasteful activation of the motor is prevented because the supply of electric power is discontinued after the elapse of the predetermined time period, but if the door is repeatedly closed and opened after the timer has already started running, the other end of the seat belt will reciprocate, and upon elapsing of the predetermined time period, the movable end of the seat belt may be located at an intermediate point of its full stroke to a considerable inconvenience of the vehicle occupant.
To eliminate this inconvenience, Japanese utility model publication No. 60-39302 proposes the provision of a pair of timer circuits which are set and reset in a mutually exclusive manner so that one of the timer circuits is reset when the other timer circuit is set. According to this proposal, since each of the timer circuits is initialized or, in other words, starts running from the start every time the door is either closed or opened, the motor keeps rotating for a predetermined time period following the detection of the last door opening or closing action, and either one of the timer circuits continues to drive the movable end of the seat belt to one of the terminal ends of the stroke thereof without fail.
However, this proposal has the disadvantage that two timer circuits are required for each passive seat belt system. Since a timer circuit requires a relatively large capacitor or complex circuitry to define a period of time such as 15 seconds which is longer than the normal time period for the moveable end of the seat belt to travel from one end of its stroke to this other, and such circuitry tends to be large in size and high in cost.